This invention relates to multilayer semiconductor devices and, in particular, to multilayer semiconductor devices with a common contact to a plurality of conducting layers providing two or more paths of current flow.
Semiconductor devices, such as transistors, can consist of multiple layers of semiconducting or insulating materials. In a vertical transistor there are at least three layers including an emitter, base and collector. Charge carriers, either holes or electrons or both, pass from the emitter, through the base, to the collector. A metallic contact is deposited on each of the layers for operating the transistor.
Horizontal or field effect transistors are also multilayer devices. The source, gate and drain each have a metallic contact. The gate may include a Schottky barrier that creates a depletion layer to control the transfer of charges that flow underneath the gate from the source to the drain.
Recent developments in semiconductors have produced multilayer heterojunction devices. These devices, either vertical or horizontal transistors, are typically fashioned from alternate layers of gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs). Under voltages suitably applied to such a device, a thin, high mobility region can be formed at the junction of such materials. This region, in a horizontal transistor, is termed a two dimensional electron gas or 2 DEG. Electrons that make up a 2 DEG have a very high mobility. Such high mobility gives such transistors a switching speed almost four-times as fast as other transistors.
In horizontal high mobility devices, such as field effect transistors, it is known to place a source and drain contact on a common upper surface. A Schottky barrier, as a gate, is positioned between the source and drain and on the upper surface. The gate controls the flow of charges in the 2 DEG between the source and drain by depleting the 2 DEG beneath the gate electrode. See, for example, U.S. Pat. Nos. 4,558,337; 4,471,367 and 4,455,564. In these devices, the source and drain contacts extend only to the upper conducting layer.
It is desirable in some applications, e.g., oscillators, to switch the conduction path between different conducting layers. Prior art horizontal devices are known having a pair of contacts common to several conducting layers of the device. A Schottky barrier is provided to sequentially deplete the conducting layers. However, current flows through all undepleted layers and there is no means for switching the flow of current between two or more layers.
Vertical high mobility devices are more difficult to fabricate. They comprise very thin layers that have to be precisely deposited or etched to expose lower layers for forming ohmic contacts. Two hundred angstroms is typical of the thickness of some layers. Molecular beam epitaxial deposition equipment can deposit such thin layers. However, it is difficult to fix metal contacts on devices with such thin layers. Therefore, a simple contact scheme is needed for devices having vertical transport.